1. Field of the Invention
The present invention relates to a method for manufacturing a liquid crystal display (or "LCD") using a thin film transistor (or "TFT"), and the structure of the LCD. Especially, the present invention relates to a method for manufacturing the TFT having a twice anodized gate electrode.
2. Description of the Related Art
LCDs have been used in a miniature display devices, such as sub-miniature TVS and display devices for battery driven note-book computers, because they consume less electric power than other displays do. LCD color displays are formed by combining liquid crystal panels with color filters. Recently, research efforts have focused on color displays, resulting in many LCD color displays being brought to market. The active matrix driving method is the most common method used to drive an LCD display. Displays driven in this fashion are known as AMLCDs (active matrix liquid crystal displays) and comprise thin film transistors as switching elements for each pixel element of the display. AMLCDs are expected to replace cathode ray tubes ("CRTs") in many video applications.
FIG. 1 is a plan view showing an AMLCD substrate including TFT switching elements.
The AMLCD includes pluralities of gate 13 and source 11 bus lines extending horizontally and vertically, respectively. A gate electrode 3 protruding from the gate bus line 13 is formed near the intersection of each source bus line 11 and each gate bus line 13. Further, a source electrode 1, extending from the source bus line 11, overlaps one side portion of the gate electrode 3, while a drain electrode 17 overlaps an opposing side portion of gate electrode 3. Thus, a TFT 15, comprising gate electrode 3, source electrode 1, and drain electrode 17, is formed at each intersection of a gate bus line 13 and a source bus line 11. Rectangular areas defined by the intersecting source 11 and gate 13 bus lines are referred to as the "pixels" of the LCD. In the pixel area, a pixel electrode 19 is electrically connected to the drain electrode 17.
FIG. 2 is a cross-sectional view of a TFT 15 and pixel electrode taken along line II--II of FIG. 1. As indicate above, the TFT includes gate electrode 3 formed on a transparent glass substrate 21. On the surface of the gate electrode 3, an anodized layer 33 is formed to improve performance thereof. A gate insulating layer 23 is formed on gate electrode 3 followed by a semiconductor layer 25 made of a-Si. An extrinsic semiconductor layer 8 of n+a-Si is formed on selected portions of semiconductor layer 25. Source electrode 1 and a drain electrode 17 are formed in contact with the extrinsic semiconductor layer 8 and pixel electrode 11, respectively.
FIG. 3 is a cross-sectional view of an AMLCD storage capacitance electrode taken along a line III--III of FIG. 1, and FIG. 4, is a cross-sectional view taken along a line IV--IV of FIG. 1, further illustrating the storage capacitance electrode structure.
As shown in FIG. 3, the storage capacitance electrode 13' is formed by overlapping gate bus line 13 with pixel electrode 19. As shown in FIG. 4, overlapping portions of pixel electrode 19 and gate bus line 13 constitute respective electrodes of the storage capacitance electrode 13' having capacitance Cs. Storage capacitance electrode 13' is provided in order to insure proper charging of the pixel electrode.
In particular, when the TFT 15 is turned off, the source electrode 1 and drain electrode 17 do not remain entirely isolated from one another, but are equivalent to resistors with relatively high resistances. Accordingly, when the TFT 15 is turned off, charge stored on pixel electrode 19 leaks through the TFT 15 in accordance with a characteristic time-constant proportional to the product of the resistance of the source/drain and capacitance associated with pixel.
As a result, it is necessary to periodically refresh or restore electrical charge to pixel electrode 19. However, if the refresh is performed too often, the quality of the LCD is deteriorate. Accordingly, if the LCD is refreshed relatively infrequently, the quality of the LCD is improved. By providing the storage capacitance electrode, charge can be supplied to pixel electrode 19 to compensate for the leaked charge without frequent refresh.
A process for manufacturing a conventional LCD comprising the TFT and the storage capacitance electrode will now be described with reference to FIGS. 5A to SF, which show cross-sectional views correspond to FIGS. 2 and 3 at stages of the manufacturing process.
As shown in FIG. 5A, an Al layer is deposited on the entire surface of glass substrate 21, and patterned to form gate bus line pad 30, the gate bus line 13, gate electrode 3 branching out from gate bus line 13. However, the surface of the Al layer can contain defects, such as hillocks. Accordingly, an anodized layer 33 is formed on the patterned Al layer, except the gate bus line pad 30.
Gate insulating layer 23, a a:Si semiconductor layer 25 and an extrinsic semiconductor layer 8 are then successively deposited as shown in FIG. 5C. Next, the semiconductor layer 25 and the extrinsic semiconductor layer 8 are patterned (FIG. 5D), followed by a formation of separated source 1 and, drain 17 electrodes and an etch of the extrinsic semiconductor layer, thereby completing the conventional TFT switching element (FIG. 5E). An ITO layer is then deposited on the entire surface of the substrate using a sputtering method, and pixel electrode 19 is formed by patterning the ITO layer (FIG. 5F).
In the conventional method for manufacturing an AMLCD including TFTs, however, defects, such as pin-holes and over etch, can occur in the metal layer of the gate bus line by photo-resist developing solution during anodization. That is, as shown in FIG. 6, in order not to form an anodized layer on the gate line pad(30) when the gate bus line (13) is anodized, the gate line pad(30) is protected by a mask(50) made of photo-resist patterned using a photolithography process. As is well known in such processes, a photo resist layer is exposed and developed using a developer solution. Frequently, however, the developer solution can attack the surface of the gate bus line(13) or the edge of the mask(50) for protecting forming the anodized layer on gate bus line pad(30).